With the constantly developed scientific technologies, the use of robots has become highly popular among different industrial and commercial fields. There are many different types of drive mechanisms designed for robots. One of the most common robots is a wheeled robot provided with one or more wheels. Usually, for the purpose of keeping static and dynamic balance, the conventional wheeled robots have three or more wheels each. In the case of having a large number of wheels, the robot usually requires a relatively large turning radius to move to another direction. Thus, while a robot with more wheels can be accepted for use outdoors, it fails to move smoothly when being used indoors, such as in a house, because the paths available in the house for the robot are usually narrow and complicated.
To enable the robot to easily move along the narrow and complicated indoor paths, other types of robots capable of keeping static and dynamic balance have been developed, including two-wheeled robots employing the inverted pendulum principle, single-axle-driven one-wheeled robots with an ellipsoid wheel, and one-wheeled robots with inverse mouse-ball drive.
The two-wheeled robot is uneasy to change its moving direction because the wheels thereof are set to a fixed direction. For a two-wheeled robot to move in a new moving direction, the robot must first be turned to orient toward the new moving direction.
The one-wheeled robot moves via a spherical wheel, which is driven by a pair of orthogonally arranged drive rollers to roll in two directions. An idler roller is provided corresponding to each of the two drive rollers. The spherical wheel is held in place by the drive rollers and idler rollers without the risk of separating therefrom, so that the drive rollers keep contacting with the spherical wheel to drive the same to roll smoothly.
When the spherical wheel is driven by one of the drive rollers to roll on the floor, the other drive roller and idler roller orthogonal to the first drive roller are not able to rotate forward in a direction the same as the rolling direction of the spherical wheel. That is, the other drive roller and the idler roller corresponding thereto do not move but slip relative to the spherical wheel. For the spherical wheel to roll smoothly, the spherical wheel must have a relatively high friction to assist in the driving by the drive roller. However, the spherical wheel must also have a relatively low friction to allow the slipping between it and the other drive roller and idler roller. In designing the spherical wheel, it is always desirable to have high friction between the spherical wheel and the floor to avoid slippage of the spherical wheel on the floor. The high friction of the spherical wheel would, however, cause difficulty in slipping between the spherical wheel and the drive roller and the idler roller and accordingly, have adverse influence on the driving efficiency of the drive roller.
In conclusion, there are still many problems being encountered with in designing the drive mechanism for a robot. A robot with three or more wheels can stably maintain balance, but it requires a relatively large space to turn around and is therefore not suitable for using indoors. A two-wheeled robot requires only a shortened turning radius and is able to pivot turn using two wheels. However, whenever it is desired to change the moving direction of the two-wheeled robot, the robot must first be turned before it can move in the new direction. And, in the one-wheeled robot, the required friction between the spherical wheel and the drive rollers and idler rollers is different from the required friction between the spherical wheel and the floor, and it is impossible for the spherical wheel to have a high friction and a low friction at the same time. As a result, the one-wheeled robot usually has relatively low driving efficiency.